1. Field of the Invention
The present invention relates to an LED device which is a light emitting device using an LED chip.
2. Description of the Related Art
In general LED devices, a resin is used to seal an LED chip, which is a light emitting element, from the surroundings for purposes of protecting the LED chip. Also, from the fact that a high-power LED device for use in illumination applications self generates a large amount of heat when it is turned on, a metal is used for a board and a reflector in order to radiate this self-generated heat (see JP-2001-85748-A).
FIG. 1 is a vertical sectional view showing the configuration of an LED device described in JP-2001-85748-A. Referring to FIG. 1, this LED device comprises LED chip 1, board 2 on which LED chip 1 is mounted, and reflector 3 including a frame which surrounds side surfaces of LED chip 1 on board 2.
Then, LED chip 1 mounted on board 2, and bonding wire 4 which electrically connects board 2 with LED chip 1 are covered with sealing resin material 5. Sealing resin material 5 is fitted in the frame which forms part of reflector 3. Reflector 3 is made of a metal material such as Al, Cu and the like.
In such a configuration, the coefficient of linear expansion in the sealing resin and reflector are largely different from each other. As a result, when the sealing material for the LED chip is an epoxy-based resin, the sealing material is highly likely to crack due to the thermal expansion of each component member while the LED is turned on, or peeling is highly likely to occur on the interface between the sealing resin and LED chip or on the interface between the sealing resin and reflector, because of the high hardness of the sealing material exhibits.
Thus, as general countermeasures to the above problem, an elastic silicone-based sealing resin is employed to avoid cracks and interfacial pealing even if each component member thermally expands.
As described above, when a silicone-based resin is used for the sealing material for the LED chip in the configuration of FIG. 1, the problems of cracks and interfacial pealing are less likely to occur because the silicone-based resin exhibits a larger coefficient of linear expansion, as compared with the epoxy-based resin.
However, even if the foregoing problem is solved, this configuration suffers from another problem in which the bonding wire for supplying electric power to the LED chip is pulled and broken.
A mechanism involved in the occurrence of this problem will be described. FIG. 2 is a cross-sectional view which schematically shows the configuration of the light emitting device in FIG. 1 to describe the mechanism involved in the occurrence of the problem. In the configuration shown in FIG. 2, a silicone-based resin is fitted in reflector 3 as sealing resin material 5, and this resin covers LED chip 1 and bonding wire 4.
Since the light emitting device is configured as described above, when heat is generated while the LED is turned on, it is anticipated that sealing resin material 5 fitted within reflector 3 will largely expand in the direction in which it moves away from board 2, and consequently bonding wire 4 will be pulled and broken (see FIG. 2). This problem causes a lower reliability of the light emitting device.